Increasing demands in fuel efficiency has made hybrid systems more attractive in the automotive industry. In addition to a conventional combustion engine, an electric motor is an important part of the hybrid system. Alternating current (AC) induction motors are commonly used because they offer simple, rugged construction, easy maintenance, and cost-effectiveness. The AC induction motor includes two assemblies: a stator and a rotor. The stator is the outermost component of the motor and is composed of steel laminations shaped to form poles with copper wire coils wound around the poles. The primary windings are connected to a voltage source to produce a rotating magnetic field.
A squirrel cage rotor is a cylinder that is mounted on a shaft rotating in an induction motor. It contains longitudinal conductor bars set into squirrel slots and connected together at both ends by short rings forming a cage-like shape. FIG. 1 shows an illustration of one embodiment of a squirrel cage rotor. The core of the rotor is built with stacks of electrical steel laminations.
The field windings in the stator of an induction motor set up a rotating magnetic field around the rotor. The relative motion between this field and the rotation of the rotor induces electric current in the conductor bars. In turn, these currents lengthwise in the conductors react with the magnetic field of the motor to produce force acting at a tangent to the rotor, resulting in torque to turn the shaft and the rotor. In effect, the rotor is carried around with the magnetic field, but at a slightly slower rate of rotation. The difference in speed is called slip, and it increases with load.
The conductor bars are often skewed slightly along the length of the rotor (i.e., the conductor bars are not perpendicular to the plane of the end ring where the end ring attaches to the conductor bars) to reduce noise and to smooth out torque fluctuations that might result in some speed variations due to interactions with the pole pieces of the stator, as shown in FIG. 1. The number of bars on the squirrel cage determines to what extent the induced currents are fed back to the stator coils and hence the current through them. The constructions that offer the least feedback employ prime numbers of bars.
The iron core (laminate steel stack) serves to carry the magnetic field across the motor. The structure and materials for the iron core are specifically designed to minimize magnetic losses. The thin laminations (steel sheets), separated by varnish insulation, reduce stray circulating currents that would result in eddy current loss. The material for the laminations is a low carbon, high silicon steel with several times the electrical resistivity of pure iron, further reducing eddy-current loss. The low carbon content makes it a magnetically soft material with low hysteresis loss.
The same basic design is used for both single-phase and three-phase motors over a wide range of sizes. Rotors for three-phase motors will have variations in the depth and shape of bars to suit the design classification.
A common aluminum rotor construction method is to cast an aluminum alloy into the laminated steel slots and simultaneously cast end rings creating an electrical circuit. Cast aluminum rotors (bars and end rings together) can have low mechanical properties, in particular, the low electric (pure Al: IACS 62%) and thermal conductivity of aluminum alloys, particularly when the cast aluminum conductor bars have casting defects including hot-tear cracks, porosity, and oxide inclusion etc, impose a great challenge for their successful application in electric motors. In addition, the aluminum alloys used to cast rotor squirrel cages are usually high purity aluminum, high purity aluminum casting alloys, or electric grade wrought alloys which are all difficult to cast because of their low fluidity, high shrinkage rate (density change from liquid to solid), high melting temperature, and large freezing range (temperature difference between liquids and solids), etc. These characteristics of the higher purity aluminum alloys increase porosity and the tendency of hot tearing, particularly at the locations where the conductor bars connect to the end rings, which leads to fracture between the conductor bars and the end rings. Furthermore, many cast aluminum squirrel rotor cages are made by high pressure die casting process in order to fill the thin and long bars (squirrel slots) in the laminate steel stack quickly to avoid cold shuts. The entrained air and abundant aluminum oxides produced during the high pressure die casting process, which are due to very high flow velocity (about 60 m/s) in mold filling, can not only decrease rotor quality and durability, but also significantly reduce the thermal and electric conductivity of the rotor, particularly in the conductor bars. In practice, it is often seen that the electric conductivity of the cast aluminum rotor (casting conductor bars and casting end rings) is only about 40 to about 45% IACS. Because of the casting defects present in the cast aluminum conductor bars, the bars may be broken during motor operation. The broken bars will further reduce rotor conductivity and motor performance.
Typically, either pure aluminum (99.7% purity), which has high electrical conductivity but low mechanical properties, or A6101-T61 alloy (0.6% Mg-0.5% Si), which has relatively high conductivity (59%) with improved strength, are used for rotors. The material's composition, electrical conductivity, and static/cyclic fatigue strength are very important to the motor's performance and durability. Therefore, high integrity wrought aluminum conducting bars and end rings are considered as replacements to the cast aluminum rotors.
Another process for making squirrel rotor cages involves inserting aluminum conducting bars through the slots in the lamination stack, rotating (or skewing) the steel lamination stack to produce slot skew (if a skewed rotor design is desired), and joining the aluminum end rings to the aluminum conductor bars by a solid state welding process.
Solid state welding is a group of welding processes which produce interfacial coalescence (joining) at temperatures essentially below the melting point of the base materials being joined, without the addition of filler metal. Pressure may or may not be used depending on the particular process. This group of welding processes includes for example, cold welding, diffusion welding, forge welding, friction welding, hot pressure welding, roll welding, and ultrasonic welding.
With some welding processes, such as friction welding, where one piece is stationary and the other piece is continuously rotating, the amount of friction force and heat generated can cause a large plastic deformation. This can result in a change in the position of the conductor bars during welding, as illustrated in FIG. 2, which is undesirable. The squirrel cage rotor 10 includes a pair of aluminum end rings 15 and the laminate steel stack 20. There are aluminum conductor bars 25 in the slots of the laminate steel stack 20. The dotted lines 30 show the original (and desired) position of the conductor bars. However, the severe plastic deformation in traditional friction welding causes the bar ends to be moved to the final position 35 which is undesired.
Friction welding is a solid state welding process which produces coalescence of materials by the heat obtained from the mechanically induced sliding motion between rubbing surfaces. The work parts are held together under pressure. The process usually involves continuously rotating one part against another to generate frictional heat at the junction as shown in FIG. 3. When a suitable high temperature has been reached, rotational motion ceases, additional pressure is applied, and coalescence occurs.
There are several variations of the friction welding process. In one, one part is held stationary, and the other part is rotated to a constant rotational speed. The two parts are brought into contact under pressure for a specified period of time with a specific pressure. Rotating power is disengaged from the rotating piece, and the pressure is increased. When the rotating piece stops, the weld is complete. Either the end rings or the lamination steel assembly with the conductor bars can be rotated while the other is held stationary. Another type of friction welding is inertia welding. In this process, a flywheel is revolved until a preset speed is reached. It, in turn, rotates one of the pieces to be welded. The motor is then disengaged from the flywheel, and the other part to be welded is brought in contact with the rotating piece under pressure. At a predetermined time, the rotational speed of the part is reduced, the flywheel is brought to an immediate stop, and additional pressure is provided to complete the weld.
Frictional welding has a number of advantages. It allows the production of high quality welds in a short cycle time. No filler metal is required, and flux is not used. The process is capable of welding most common metals, and it can be used to join many combinations of dissimilar metals. It can be used with 6101-T61 alloy components that have been processed with a solution heat treatment and aged to provide optimum strength and conductivity. It also requires minimum heat input which will generate a minimum heat affected zone and have little influence on conductivity and material properties.
However, friction welding requires a relatively expensive apparatus.
Another solid welding process is ultrasonic welding, which produces coalescence by the local application of high-frequency vibratory energy as the work parts are held together under pressure. Welding occurs when the ultrasonic tip (or sonotrode), the energy coupling device, is clamped against the workpiece and is made to oscillate in a plane parallel to the weld interface. The combined clamping pressure and oscillating forces introduce dynamic stresses in the base metal. This produces minute deformations which create a moderate temperature rise in the base metal at the weld interface. This, coupled with the clamping pressure, provides for coalescence across the interface to produce the weld. Ultrasonic energy will aid in cleaning the weld area by breaking up oxide films and causing them to be carried away.
The vibratory energy that produces the minute deformation comes from a transducer which converts high-frequency alternating electrical energy into mechanical energy. The transducer is coupled to the workpiece by various types of tooling, including tips similar to resistance welding tips. The normal weld is a lap joint weld.
The temperature at the weld is not raised to the melting point, and therefore, there is no nugget similar to resistance welding. Weld strength is equal to the strength of the base metal. Most ductile metals can be welded together, and there are many combinations of dissimilar metals that can be welded.
As illustrated in FIGS. 4-5, the work pieces 110, 115 are placed between a fixed machine part, e.g., the anvil 120, and the ultrasonic weld head 105 oscillates horizontally during the welding process at high frequency (usually about 20, about 35, or about 40 Hz). Static pressure is applied normal to the welding interface. The pressure forces are superimposed by the high-frequency oscillating shearing force. As long as the forces in the work pieces are below their elastic limit, the pieces will not deform. The shearing forces at high frequency break down the surface oxide layer, remove it, and produce a bond between the pure metal interfaces. The circular assembled rotor could be rotated (indexed) to weld each bar to the end rings, or it may be possible with a multi-head tool to weld both end rings to the ends of each conducting bar simultaneously.
Another solid state welding process is laser welding, as shown in FIG. 6. Laser welding does not add any filler, and it is a pure fusion weld process. A Nd:YAG (neodymium:yttrium-aluminum garnet) laser is preferred because it is more compatible with aluminum than a CO2 laser. The wavelength of a YAG laser is extremely short (1.064 nm), which readily couples with the highly reflective aluminum surfaces. Typically, two beams would be used in a single laser so that the metal remains molten longer and fills the weld more consistently. Lasers with power ranging from two to eight kW would be beamed through the two lenses. The two beams per laser help to overcome the tendency of aluminum to cool rapidly. The pure aluminum or alloyed conducting bars would be inserted through the laminated stack slots and into slotted end rings. The twin beam YAG laser system would then weld the conducting bar to the end ring from top and side positions. Two lasers could be used, one (A) to weld the top of the conducting bar to the end ring 15, and one (B) to weld the side of the bars to the end ring. Using laser welding, each conductor bar would have to be welded separately, increasing the time required to form the squirrel cage.
In the various welding processes, time, temperature, and pressure, individually or in combination, produce coalescence of the base metal without significant melting of the base metal. Some of the processes offer certain advantages because the base metal does not melt and form a nugget. The metals being joined retain their original properties without the heat-affected zone problems involved when there is base metal melting. In some processes, the time element is extremely short, e.g., in the microsecond range or up to a few seconds.
Therefore, there is a need for an improved rotor for an electric machine, and for methods of making improved rotors.